To date, for most solar water heaters, both those using flat-plate collectors and those using vacuum-tube collectors, hot-water storage tank is an essential component, because sunlight is intermittent. To keep the water hot for hours and days without sunlight, tank size must be sufficiently large, typically 100 liters to 250 liters. For convection-operated systems, the tank must be placed above the solar radiation collector. The weight of the tank is acting on the roof as a concentrated mechanical pressure. If the heat storage tank is placed no higher than the heat collector, an electric pump is necessary. The electrical pumps, control units, connecting pipes and valves between the solar thermal energy collector and the heat-storage tank are expensive, and require frequent service and maintenance.
The vacuum tubes as solar radiation collectors are also excellent heat storage devices because of the vacuum sleeve. To one with an ordinary skill in the field of solar water heaters, it is obvious that vacuum tubes can be used to store energy. Therefore, an array of vacuum tubes alone could function as a complete solar water heater without a hot water storage tank. Such a design is advantageous over the solar water heaters with a tank: Comparing with the integrated convection-driven systems, the overall structure is simplified, and the mechanical pressure on the roof is reduced and evenly distributed. Comparing with the separated systems, the electricity-powered pump and control unit can be eliminated; therefore the system can run maintenance free. However, by using sensible heat of water to store thermal energy, the volume must be very large. Vacuum tubes of very large diameter must be used. Therefore, in order to build a tankless solar water heater, two problems must be resolved. First, to find a type of thermal-energy storage medium with the following properties: (1) having a heat capacity much greater than water in the upper range of water temperature; (2) having no hysteresis or incongruent phenomenon during heat cycling; (3) inexpensive; and (4) nontoxic. Second, to design a water handling apparatus to transfer the thermal energy stored inside the vacuum tubes to running water with the following properties: (1) with no need of an electrical power; (2) automatic control based on the nature of materials; (3) ensuring the highest possible efficiency; (4) minimizing heat loss; (5) inexpensive to produce; and (6) easy to install. It is the goal of the current invention to resolve the above problems to construct tankless solar water heaters using vacuum tubes.